Plunger pumps consist of a so-called "power end" and a "fluid end". Typically the power end consists of a crank shaft, and a plurality of connecting rods, coupled to pistons which, in turn, are carried in guide sleeves or cylinders. To this extent such power ends resemble the design of a typical automotive engine.
The fluid end assemblies are attached to the power end, in the place where the cylinder head would normally be found in an automotive engine. Typically such fluid ends consist of a cylinder or pressure chamber having suitable inlet and outlet ports with valves, and a plunger adapted to reciprocate in the cylinder. The plungers are coupled to the pistons in the power end.
Pumps of this general type are available from a variety of different manufacturers. It will, of course, be appreciated that such pumps are multi-cylinder pumps, and there will be one fluid end for each cylinder. In many cases, the fluid ends are manufactured as a single assembly, or are mounted on a common mounting plate, which is then attached directly to the cylinder block of the power end. In most cases, the power ends are manufactured by one manufacturer, and the fluid ends are manufactured by another, and are then assembled together. Certain manufacturing tolerances are permitted in the manufacturing of the power ends, and also the fluid ends. Where these tolerances do not precisely match, then it may cause difficulties in matching or mounting the fluid end assembly to the power end assembly. This is not simply a matter of allowing for tolerances in the location of fastening holes and the like. This itself would present little or no problem. The problem is that the plungers of the fluid ends are mounted on the pistons in the power ends. If the centres of the pistons do not precisely match the centres of the fluid end cylinders, then the plungers will be slightly offset from the axis of the fluid end cylinder.
This will then cause a high degree of wear, leading to failure of fluid seals and breakdown of the pump.
Another separate problem in the design of such fluid ends, lies in the manner in which they are bolted to the cylinder block of the power end. During the pumping action of the plungers, very substantial hydraulic forces are developed in the fluid end cylinders. In past designs, a system of bolts was provided clamping the entire fluid end cylinder assembly directly on to the power end cylinder block. As a result, all of the stresses created by the hydraulic forces developed in the fluid end cylinders were carried directly through these hold-down bolts, to threads formed in the cylinder block of the power end. The engineering of these hold-down bolts and the threads in the block had to be sufficient to withstand these stresses.
This problem of stresses generated in the fluid end is more complex than it appears. During the power stroke, the force developed by the plunger is calculated by multiplying the maximum pounds per square inch of hydraulic pressure by the area of the piston head. This force is clearly transmitted back to the cylinder block of the power end, and the cylinder block of the power end must be engineered so as to withstand it. In practice this type of engineering is well understood and is generally speaking managed in a satisfactory manner.
However, the fluid end consists of two components mainly the cylinder itself, and the valve housing attached to the top of the cylinder. Fluid seals, typically O-rings, are located between the valve housing and the cylinder. These O-rings must necessarily be located in grooves which are spaced radially outwardly from the cylinder. The force that is developed on the valve housing will, therefore, be calculated by multiplying the maximum pounds per square inch of hydraulic pressure by the area of the valve housing contained within the O-rings seal. Even though the O-ring seal is only slightly spaced radially outwardly with respect to the cylinder, this area may easily be fifty to one hundred percent greater than the area of the piston head.
Consequently, the force applied to the valve housing may be double that applied to the piston head. In the usual design of hold-down bolts, this means that the hold-down bolts, and their receiving threads in the cylinder block of the power end, must be engineered so as to withstand twice the force imposed on the piston head. This greatly increases the difficulty of engineering a satisfactory power end, and a satisfactory system of hold-down bolts, and in fact, is a frequent cause of problems in such plunger pumps.
Another problem in relation to plunger pumps lies in the servicing of the fluid ends. In use, it is necessary to strip down and service the fluid end from time to time, to replace fluid seals, and other worn parts. If, for example, when a pump is in use on a job site, one of its fluid ends develops a problem, it was generally necessary to remove the entire fluid end assembly for servicing. This caused considerable downtime, and possible entry of contaminants into fluid end cylinders which were otherwise sound.